All optical-photonic switching and signal processing devices are being developed to exploit speed and bandwidth advantages of fiber optics. These devices are divisible into two distinct categories: highly parallel devices for complex tasks that may be relatively slow such as data array manipulation, and ultrafast simple devices for serial processing applications such as front/back end communications processing.
For ultrafast applications, optical fibers present an attractive medium in which to perform switching. Fibers have a very short response time for the nonlinearity and have a very low absorption coefficient.
In addition to providing high speed, it is also necessary to have incremental or small-signal gain. Devices which exhibit such incremental gain have been important to various switching, computing and signal processing applications. In a classical example, the incremental gain of transistors permits relatively small signals to control other much larger input signals, By providing incremental gain, it is possible to cascade devices so that one output may be fanned-out to several inputs. These devices have been somewhat harder to obtain in the optical domain.
Recently for optical devices, it was shown that a nonlinear fiber interferometer possesses the ability to operate as a switching device with some incremental gain. See S. Shirasaki et al., Proceedings of CLEO 1987, page 187. Shirasaki et al. designed a single fiber interferometer in which two mutually orthogonal polarized versions of the same probe pulse are delayed with respect to each other and interfere under control of a cotraveling control pulse. The fiber is standard circular core optical fiber. The control pulse changes the index seen by the people pulse so that the polarization of the probe pulse is changed upon recombination. Shirasaki et al. rely upon the nonlinear index, n.sub.2, in the fiber to provide gain for the device. However, this arrangement is severely limited in the amount of gain it can possibly achieve because it requires the power in the control signal to be substantially equal to the power in the switched probe signal to achieve significant output power.